AUDIO AMPLIFIERS

Abstract: An audio amplifier has a relatively low RMS power capability, so reducing the size and cost of the power components and associated heat sinks, but a relatively high peak power capability determined by the power supply voltage. Audio quality, especially for musical performances, is therefore good. An attenuator (12) is positioned before the power amplifier (18), controlled by an RMS detector (14) and threshold detector (16). Signals which after amplification would exceed the RMS power capability are attenuated.

Description: AUDIO AMPLIFIERS This invention relates to audio amplifiers and in the most important example to audio power amplifiers capable of delivering high power audio signals to loud speakers. It is usual to rate audio power amplifiers in terms of the maximum root-mean-square (RMS) power levels that the amplifier is capable of delivering to a defined load impedance in response to a continuous sine wave input. Thus a typical power amplifier may be rated at 400 watts at 2 ohms. As will be recognised, the maximum peak power level will exceed the RMS value by around 3 decibels. Audio power amplifiers are typically expensive and relatively heavy. This is as a result principally of the cost and weight of power transformers, the cost of high rated power transistors and the weight of the associated heat sinks. Considerable efforts have been made to reduce the cost and weight of audio power amplifiers and reference is directed for example to WO 80/01023. Techniques disclosed in this reference and elsewhere include control of the duty cycle of the primary transformer winding to enable the use of smaller transformers; and switching between high and low voltage supply rails in dependence upon the audio level, so as to reduce dissipation in the power transistors. The switching can be performed in a variety of ways and suggestions for improvement are to be found in US 4,339,730 and US 4,498,057. It nonetheless remains the case that significant increases in the output power level of audio amplifiers have only been achievable at considerable cost. In the amplification of electronic instruments for public performance, there is a requirement for substantially increased power levels which can with existing technology only be met at very considerable cost. It is one object of this invention to meet the requirement for high power audio amplification using circuitry of substantially reduced costs. It is another object of this invention to improve the performance, through increasing the power capability, of audio amplification equipment within a particular price grouping. It has been recognised that, in the amplification of electronic musical instruments, the power rating that is of primary importance in creating the desired "sound" is not the RMS, (or any other average figure) but the peak power capability. Conventionally, power ratings have been considered in the context of continuous sine wave input signals. Peak and RMS power values are in this context in the fixed ratio of around 3 decibels. If, on conventional thinking, a particular application demanded a peak power capability of 2000 watts, an audio amplifier circuit would be designed having an RMS power capability of 1000 watts. Even with the techniques that have been mentioned for reducing power dissipation within the amplifier circuitry, construction of an audio power amplifier having an RMS power requirement of 1000 watts involves substantial costs. The present inventor has recognised that audio signals, at least those generated by electronic musical instruments, are very far from sine waves and the present invention provides an improved audio amplifier, the design of which is not constrained by a theoretical relationship between peak and RMS power rating. Accordingly, the present invention consists, in one aspect, in audio amplifier means comprising signal conditioning means for receiving an input audio signal and delivering a conditioned audio signal; voltage supply means for providing a maximum supply voltage V™ ax ; and power amplifier means having an input terminal connected to receive said conditioned audio signal, a power terminal connected to said voltage supply means and an output terminal for connection to a loudspeaker or other load having a defined impedance, to deliver an output audio signal of peak power W p and RMS power W^; wherein the power amplifier means is capable of delivering a maximum RMS power level W πns ax , characterised in that the power amplifier means is adapted through selection of V"" such that the maximum peak power W p mϊ exceeds W m ,. 11 " 1 ' by at least 5 decibels and in that said signal conditioning means includes attenuation means for attenuating input signals which would otherwise on amplification produce an RMS power level exceeding W mB m*x . Advantageously, the attenuation means comprises an RMS detector serving to detect an RMS value for the input audio signal and a variable gain amplifier receiving the input audio signal and having its gain controlled in response to the output of the RMS detector. Suitably, the power amplifier means is adapted so that the maximum peak power W p m x exceeds the maximum RMS power W πns m x by at least 10 decibels. In another aspect, the present invention consists in a method of amplifying an audio signal having peak values substantially in excess of time-averaged values, comprising the steps of providing a power amplification stage having a peak power capability substantially greater than its average power capability and presenting to the power amplification stage a modified audio signal from which have been removed components having average values such as to cause the amplification stage to exceed its average power capabilities. By breaking the conceptual link between peak and RMS power ratings, the present inventor has enabled the production of a power amplifier having, in one example, an RMS power rating of 400 watts yet a subjective performance which, in terms of the sound produced, is comparable with an audio amplifier of traditional design having an RMS power rating of 2000 watts. Thus, an audio power amplifier according to this invention with an RMS power rating of 400 watts can provide a peak power of 4000 watts. To produce a peak power capability of this value within conventional design constraints would require an RMS power rating of 2000 watts. Whilst a power amplifier according to this invention having an RMS rating of 400 watts is not necessarily as economic to manufacture as a conventional 400 watt amplifier, it is considerably more economic to manufacture than a conventional 2000 watt amplifier. The present invention takes advantage of the fact that many power limiting features are related to RMS power levels. Particularly, the problem of heat dissipation. Peak power capability , on the other hand is governed largely by the available voltage swing and although this in itself carries problems of power dissipation, these problems can be substantially overcome using known techniques. For example, the technique mentioned above of utilising two or multiple supply rails controlled so that the maximum supply voltage is applied only across the power transistors when the required power output voltage approaches the maximum supply voltage should maintain power dissipation within acceptable limits. The invention will now be described by way of example with reference to the accompanying drawings in which :- Figure 1 is a block diagram illustrating an audio power amplifier according to the present invention; and Figure 2 is a more detailed block diagram illustrating an RMS detector for use in the circuit of Figure 1; Figure 3 is a circuit diagram showing part of a circuit according to the present invention, incorporating the RMS detector, threshold detector and voltage controlled amplifier of Figure 1; Figure 4 is a circuit diagram showing a power amplifier stage for use with the circuit of Figure 3; and Figure 5 is a block diagram illustrating an audio power amplifier according to a further embodiment of the present invention. Referring initially to Figure 1, the input audio signal is taken through terminal 10 to a voltage controlled amplifier 12. The input audio signal is also presented to an RMS detector 14 which provides, through a threshold detector 16, an output to the gain control terminal of the amplifier 12. The threshold detector 16 compares the output of the RMS detector 14 with a reference which is representative of the maximum RMS value of the input signal which can be accommodated without exceeding the RMS power output capability of the amplifier. If the RMS value of the input signal exceeds this threshold, an output is generated from the threshold detector 16 causing the gain of the amplifier 12 to drop beneath unity. The output from the voltage controlled amplifier 12 is passed to a power amplifier 18. This has a bi-polar multi-rail power supply shown schematically at 18. This receives a control input from the output of the power amplifier 18, on terminal 22. This control input is used, as will be described in more detail, to vary the power supply voltage as the audio level changes. The power amplifier 18, particularly in regard to the maximum voltage available from the power supply 20, is arranged to deliver high peak power, in one example 4000 watts. The amplifier 18, however, is incapable of handling a continuous sine wave having this peak power level and indeed has a maximum RMS power capability of 10 decibels beneath the peak power rating, that is to say 400 watts. In accordance with this invention, the input to the power amplifier is conditioned by the combined effect of the RMS detector 14 the threshold detector 16 and the voltage controlled amplifier 12 to ensure that the amplifier cannot receive an input signal which would cause the RMS power rating to be exceeded. Because of the nature of audio signals derived from guitars and other electronic musical instruments, the subjective quality of sound produced by the described power amplifier is, however, comparable with an amplifier according to prior art design having an RMS power capability which is up to a factor of five times greater. As will be appreciated, a reduction of this order in the RMS power capability enables significant cost and weight reductions to be made in - for example - the size and rating of transformers and heat sinks. Turning to Figure 2, the RMS detector 14 is shown in greater detail . In a strict mathematical computation of RMS values, the time integral of the square of the signal is integrated over time. In a practical arrangement, an exponential weighting is applied. Thus, the output from an absolute value detector 30 is taken through a squaring circuit 32 to a logarithmic detector 34. A mean computation is performed in block 36 with the result passing through square root circuit 38 to the output of the RMS detector. Turning now to Figure 3, a more detailed circuit arrangement is provided. An audio input signal on terminal 50 is taken through capacitor Cl and resistor Rl to pin 1 of a voltage controlled amplifier in IC1 and through capacitor C2 and resistor R2 to pin 1 of an RMS detector in IC2. The RMS detector receives biasing and reference signals through resistors R3, R4, R5 and capacitors C3 and C4. These establish the intersection and slope of the output appearing on pin 7. Supply voltage is provided to pin 8 through resistor R6 and capacitor C5. The output on pin 7 is logarithmically related to the detected RMS value with reference to the preset maximum RMS level of the input. With the maximum RMS input level being set at that corresponding with an output into 2 ohms of 400 watts, the output on pin 7 goes positive as the input RMS level increases, at a rate of 6.1 mV/dB. In this described embodiment, IC2 is that commercially available as "IC RMS-Level Detector 2252" from THAT Corporation, Natick, Massachusetts, USA. The output on pin 7 is taken through a precision rectifier shown generally at 54 and an amplification stage shown generally at 56, before being presented as a control input at pin 3 of IC1. The detail of the circuits 54 and 56 are not germane to the present invention and further description is not required. In this described embodiment, IC1 is that commercially available as "IC voltage controlled amplifier - 2155" from THAT Corporation, Natick, Massachusetts, USA. An output is provided at pin 8 which corresponds to the audio input at pin 6 amplified with a gain determined logarithmically by the control voltage at pin 3. In this arrangement pin 7 is connected to a positive supply rail, pin 5 is converted through resistor R7 to a negative supply rail and pins 2, 4 and 8 are grounded. The output of ICI on pin 8 is taken through a virtual earth circuit shown generally at 58, to output terminal 60. The detail of the circuit 58 is not germane to the present invention and further description is not required. It will be understood that the circuit of Figure 3 operates in two different modes in dependence upon whether the detected RMS level is above or below that which corresponds with the maximum RMS power level W πns m of the subsequent power amplification stage. The interval over which the signal is integrated to calculate the RMS value can be varied and is in this example 35 milliseconds. Below the RMS threshold, audio signals are passed without attenuation, even though the instantaneous amplitudes may correspond with peak power levels 5 or even 10 decibels above the RMS power rating of the power amplifier. Above the RMS threshold, that degree of attenuation is introduced which is the minimum necessary to ensure that the maximum RMS power rating W^" 1 ™ is not exceeded. In a modification, not shown, the amplification stage 56 is omitted and the output from the precision rectifier 54 is taken to pins 2 and 4 of the voltage controlled amplifier of ICI, that is to say the inverse control port with pin 3 then being grounded. A variety of still further modifications will occur to the skilled man. Referring now to Figure 4, there is shown the power amplifier stage having positive and negative channels, each in this embodiment including seven cascaded power transistors. The power transistors are referred to generally as T R P+ and T R P" and may for present purposes be treated as a single power transistor in each channel. The conditioned audio signal at terminal 60 of the Figure 3 circuit is connected between the base inputs of the cascaded power transistors T R P+ and T R P" . The emitters of the two channels are connected to output terminal 80; the collectors are connected through respective diodes Dl and D2 to low tension rails +LT and -LT respectively. The collectors are additionally connected to respective high tension rails +HT and -HT through switch arrangements which will now be described. For convenience, only the switch arrangement for the positive channel will be discussed; the arrangement for the negative channel is an exact analogue. FET1 has its source connected to the output terminal 80 through resistors R31 and diode D4, its drain connected through resistor R20 to the +HT rail and its gate connected to the +LT rail. The junction of resistor R20 and the drain of FET1 is taken through resistor R22 to the gate of FET2, having its drain connected to the +HT rail and its source connected through resistor R23 with the common collectors of power transistors T R P+ . The source of FET2 is further connected to the base of each of tandem transistors TR1 and TR2. Each of these tandem transistors has its collector and emitter connected between the +HT rail and the common collectors of power transistors T R P+ . In operation, at low audio output levels, the gate of FET1 is positively biased with the respect to the source so drawing current through FET 1 and developing a voltage across resistance R20. FET2 is accordingly turned off so holding off both TR1 and TR2. In this situation, the common collectors of power transistors T R P+ are connected to the low tension rail +LT but are isolated from the high voltage rail +HT. As the voltage on the output terminal 80 approaches the voltage on rail +LT, FET1 begins to turn off with the voltage across resistance R20 accordingly dropping. The voltage at the gate of FET2 will in consequence rise turning on FET2 as well as tandem transistors TRl and TR2. Accordingly, over a limited range of output voltage, the voltage supplied to the power transistors will rise smoothly from that of the lower voltage rail +LT to that of the higher voltage rail +HT. As has already been explained, the advantage of this switching arrangement between voltage rails is a reduction in the collector/emitter potential of the power transistors and thus a reduction in power dissipation. The described arrangement by which there is a smooth transition between the voltage rails is felt to offer improved performance over the previously suggested arrangements in which there is either abrupt switching between voltage rails or "bootstrap" control of the power voltage. The function of diodes Dl and D2 is to isolate the power transistors from the lower voltage rail once they have been lifted to the voltage of the high voltage rail. It should be understood that this invention has been described by way of examples only. Thus, whilst there is believed to be particular advantage in combining the described signal conditioning as shown in Figure 3 with the supply rail switching arrangements of Figure 4, the signal conditioning will offer advantages in arrangements where the desired peak power capability can be achieved without the need for multiple supply rails. Additionally, the described arrangement of a voltage controlled amplifier and an RMS detector is but one of a variety of ways in which the RMS value of an input signal can be kept beneath a threshold value whilst still passing instantaneous high amplitude signal excursions. One such alternative will now be described with reference to Figure 5. In this proposal, an audio input signal at terminal 100 passes through a non-linear attenuator 102 serving to pass signals transparently beneath a threshold. The power amplifier 104 has associated with it an RMS detector 106 serving to detect, over an appropriately chosen time interval, the RMS power level. In one simple arrangement the RMS detector operates thermally to measure heat dissipation. The RMS detector 106 provides a control signal to the attenuator 102, to ensure that as the RMS level approaches the threshold, the degree of attenuation is increased. The RMS power level is thus maintained within the acceptable band, through feedback. Whilst use has been made in this description of RMS values, for the reason that these are most commonly used it should be understood that no strict mathematical definition is implied.

Claims: CLAIMS 1. Audio amplifier means comprising signal conditioning means for receiving an input audio signal and delivering a conditioned audio signal; voltage supply means for providing a maximum supply voltage V"*; and power amplifier means having an input terminal connected to receive said conditioned audio signal, a power terminal connected to said voltage supply means and an output terminal for connection to a loudspeaker or other load having a defined impedance, to deliver an output audio signal of peak power W p and RMS power W β ; wherein the power amplifier means is capable of delivering a maximum RMS power level W rms max , characterised in that the power amplifier means is adapted through selection of V"** such that the maximum peak power W p max exceeds W^"" by at least 5 decibels and in that said signal conditioning means includes attenuation means for attenuating input signals which would otherwise on amplification produce an RMS power level exceeding W E max 1 rms 2. Audio amplifier means according to Claim 1, wherein the attenuation means comprises an RMS detector serving to detect an RMS value for the audio signal. 3. Audio amplifier means according to Claim 2, wherein the RMS detector serves to detect an RMS value for the input audio signal. 4. Audio amplifier means according to Claim 3, wherein the RMS detector serves to integrate a signal value over a defined sample period. 5. Audio amplifier means according to Claim 4, wherein said sample period is between 10 and 100 milliseconds. 6. Audio amplifier means according to any one of Claims 2 to 5, wherein the attenuation means further comprises a variable gain amplifier receiving the input audio signal and having its gain controlled in response to the output of the RMS detector. 7. Audio amplifier means according to any one of the preceding claims, wherein the power amplifier means is so adapted that the maximum peak power W p ax exceeds the maximum RMS power W πns mϊX by at least 10 decibels. 8. A method of amplifying an audio signal having peak values substantially in excess of time-averaged values, comprising the steps of providing a power amplification stage having a peak power capability substantially greater than its average power capability and presenting to the power amplification stage a modified audio signal from which have been removed components having average values such as to cause the amplification stage to exceed its average power capabilities. 9. A method according to Claim 8, wherein the peak power capability of said power amplification stage exceeds the average power capability by at least 5 decibels. 10. A method according to Claim 9, wherein the peak power capability of said power amplification stage exceeds the average power capability by at least 10 decibels.

Inventors: KEIR BRUCE, KEIR, BRUCE
Applicants: KEIR BRUCE, MARSHALL AMPLIFICATION PLC, KEIR, BRUCE
Application year: 1994
Main classification: H03F1
IPC codes: H03F1, H03F3, H03G3